On the Internet, there are spaces that are moderated and meant to be safe for kids, and spaces that aren’t. Nintendo has very strict content rules and adults that play games like Splatoon are expected to follow those rules. When I was 10, my Mom found a lovely Animal Crossing fan forum that was run by a dedicated team of moderators who kept all content safe for kids. These kinds of spaces are deliberately carved out to create fannish spaces that are safe for child fans where adults are welcome to participate but must follow the content rules.
Tumblr and Ao3 are not spaces like this. Adults are allowed to post what they want so long as appropriate content warnings are attached. While Tumblr desktop is open for teenagers under 18 (the mobile app is still 17+) it’s a good opportunity for them to learn how to police their own consumption of content, respect the boundaries that adults set for that content, avoid things that bother them, and start to see people in different age groups as peers rather than authority figures. Don’t like it? Go back to a forum where there are adults who are willing to filter content for you. Think their content rules are too strict? Well, learning to moderate your own fannish experience is part of growing up and handling the maturity required for having your own freedom.
As a speech therapist/educator, I’m always trying to find simple techniques/explanations to help little kids process and understand their emotions and then teach them healthy ways to express themselves when they’re angry, frustrated, etc. I notice that telling them I deal with the same difficulties, like wanting to yell when I’m angry, goes a long way. Then they know what they feel isn’t weird BUT there are ways to handle their emotions that don’t hurt themselves or others.